In recent years, MLS cylinder head gaskets for internal combustion engines have become a preferred choice with at least two gasket layers formed from steel. In a typical MLS gasket, the layers are formed with a plurality of openings. More specifically, typical gaskets include cylinder bore openings, coolant openings, bolt holes and oil holes. Typically, the gasket includes full beads around the cylinder bore openings and half beads that surround the non-cylinder bore openings and the outer periphery of the gasket. The bolt holes, which are generally located around the periphery of the gasket, cooperate with bolts to secure the gasket between the cylinder head and engine block. The cylinder bore openings, coolant holes and oil holes are sealed by surface pressures formed by the clamping action of the bolts to deform the half or full beads, thereby creating a sealing pressure between the portions of the cylinder head and engine block that are in contact with the gasket.
Generally, when the bolts are tightened, the surface pressure between portions of the cylinder head and engine block that are in contact with the gasket varies somewhat throughout the sealing joint. Since the seal of cylinder bore openings is generally of primary concern in head gasket manufacture, other sealing portions, usually half beads, of the gasket must accommodate this priority of design. During engine operation, the high pressures and temperatures, and the operating conditions at the head gasket location cause movement between the cylinder head and the cylinder block. This movement causes a variation in sealing pressures and may cause the full beads or half beads to compress until bead portions contact a surface that inhibits further compression. This variation in sealing pressure is especially prevalent in the half beads surrounding the non-cylinder bore openings.
With respect to outer edges of the gasket, for example, to compensate for unequal surface pressures, one solution has been to provide a half bead around the periphery of the gasket. The half bead deforms and recovers with relative movement between the cylinder block and cylinder head, thereby ensuring a seal therebetween. However, during many operating conditions, some portions of the half beads becomes fully compressed, or fully deformed, between the cylinder head and engine block, and may experience a reduction of recovery capability and/or characteristics, and hence the sealing effectiveness of the half bead.
In practice, no cylinder head and cylinder block mating surfaces are perfectly planar or perfectly parallel. Gaps between the cylinder head and cylinder block exist if an engine were to be assembled without a gasket interposed therebetween. With a gasket installed between the cylinder head and cylinder block, areas of localized stress are created when fasteners (not shown) such as a bolt are positioned through bolt holes, and then tightened to secure the cylinder head and cylinder block. These gaps result in unequal sealing stresses around the bolt holes, the cylinder bore openings, and other portions of the gasket. Furthermore, the gasket will be subject to high loads in compression during engine operation. This high compression loading generates higher stresses that may result in an undesirable reduction in recovery capability and/or characteristics of the gasket, such as cracking of bead portions or plastic deformation of bead portions.
Evaluations have been performed on gaskets that have experienced an undesirable reduction in recovery capability and/or characteristics at portions, such as the peripheral edges. In many gaskets evaluated, the undesirable reduction in recovery capability and/or characteristics of half bead portions were limited to predictable peripheral areas for different applications (such as bead profile or engine type, i.e. four cylinder, eight cylinder). Accordingly, there is a need to provide a sealing mechanism that may prevent half beads from being undesirably altered during engine operation, thereby reducing sealing effectiveness or introducing potential leaks.